The present invention relates to a method and to an appliance for supplying electrical power in a mobile appliance, for example for supplying electrical power in a digital processor, and in particular in a mobile telephone. The object of the invention is to solve power supply problems that occur in portable appliances fitted with a battery whose voltage gradually falls in use and can fall below a critical value such that operation of the digital circuits of the mobile or portable appliance is no longer guaranteed.
In portable appliances in general, and in mobile telephones in particular, a battery providing a usable supply voltage Vu is connected to digital processors via a regulator. The battery can also be connected to other circuits directly if there is no need to regulate the supply voltage for those other circuits. A regulator establishes a voltage offset between the voltage at the battery terminals and some lower voltage at the output of the regulator. The output voltage of the regulator is controlled (regulated) by modulating this voltage offset; the lower the battery voltage, the lower the value of the voltage offset.
In practice the voltage offset is obtained by means of a voltage drop in a bipolar transistor or a field-effect (MOS) transistor, which has the following consequences. When the battery voltage is low, and the voltage offset, the emitter-collector voltage or the drain-source voltage, respectively, is limited, the biasing of the transistor causes a high bias current to flow into the transistor. This leads to a paradox in that as the battery is progressively discharged its voltage reduces progressively, yet the current that must be drawn from it to maintain a satisfactory supply voltage at the output of the regulator increases progressively. A second paradox occurs in this situation in that, in order to regulate the voltage, the regulator draws more current than the processor itself. As a result of this the remaining battery life of the portable appliance is quickly shortened.
A solution which solves this problem in part is to use larger transistors, instead of increasing the bias current. Apart from the fact that larger transistors are more costly, they take up more space in an integrated circuit incorporating them. This penalty is aggravated by the fact that several regulators are generally implemented in the same integrated circuit to enable tight control of power consumption in a portable appliance. The ballast transistors then take up an unacceptable amount of space.
The rise time of the regulator is too long to cope if the electrical power consumption changes suddenly at a given time. As a result there is a transient change in the regulated voltage at the output of the regulator, during which transient state the voltage first falls below and then rises above the regulated value, before stabilizing thereon. The transient response time can be of the order of 10 microseconds, especially during the period in which the voltage is falling. In a portable appliance, in particular in a mobile telephone, the digital circuits are clocked by a fast internal clock. The clock runs at 13 MHz in one prior art example. The 10 microseconds period in which there is a voltage deficit then corresponds to more than 100 processor cycle times. In some situations, especially when the battery is almost completely discharged, processes which entail an increase in power consumption start up under less than ideal conditions, because the processors are under-powered during these clock cycles.
One prior art solution to this problem is to place high-capacitance capacitors at the input or output of the regulators, preferably at the input. These capacitors can have a value of 3 000 xcexcF, for example. The disadvantage of high-capacitance capacitors is their large overall size, their cost and the cost of installing them in the mobile appliance. If different signal processors of a mobile telephone are being used simultaneously, for example a voice recognition circuit and an acoustic feedback cancellation circuit or other circuits, the high-capacitance capacitor is eventually discharged and thus of no use when it is required.
An object of the invention is to solve the above problems and in particular to prevent the regulated supply voltage dropping below a critical operating voltage at the start of the time period in which it is needed. In accordance with the invention, and in the case of GSM mobile telephones in particular, it has been noted that the times at which the electrical power demand increases occur are known in advance. Quite simply, in TDMA (time division multiple access) mobile telephone systems, the circuits of a mobile telephone are activated during only one time slot in each frame. The duration of a time slot is known (it is 577 microseconds in one example), and the time slot is repeated from one frame to the next, the frames continuing to be synchronized. It is therefore possible to tell exactly when a time slot will begin.
The invention causes the transient phenomenon affecting the regulator to occur under conditions such that it does not impede the provision of a voltage regulated to a required value during the time slot. For a time period equal to the duration of the transient phenomenon, the regulator function of the regulator is deliberately put out of adjustment in order to pre-compensate the voltage drop that inevitably occurs at the time of the increase in electrical power demand. In the same line of thinking, it is possible, before an actual increase in power consumption occurs, to provoke an increase in the power consumption of the same order of magnitude as the actual increase so that the transient phenomenon occurs before the time slot. In both cases the mode of operation of the regulator is modified in advance. It is shown below that this neutralizes the harmful effects of a sudden change of the mode of operation of the regulator.
The invention therefore provides a method of supplying electrical power to a digital processor in a mobile appliance, in which method:
a supply voltage of the processor is measured,
the supply voltage is compared to a reference, and
a control circuit supplying the supply voltage is regulated depending on the offset between the supply voltage and the reference,
the method being characterized in that:
a predictable time of an increase in electrical power demand is identified, and
the mode of operation of the control circuit is modified in advance of that time.
The invention also provides a mobile appliance provided with a digital processor and a system for supplying electrical power to said digital processor, said electrical power supply system including a circuit for slaving a power supply voltage of said processor to a reference value, the device being characterized in that it includes a predictor for predicting a time of an increase in the power consumption of the processor and means for modifying the mode of operation of the control circuit in advance.